1. Field of the Invention
The invention relates generally to manufacturing integrated circuits. More particularly, the invention relates to manufacturing integrated circuits by using an active second exposure mask having a pattern that modifies a previously exposed circuitry feature.
2. Background Information
Lithography is a step that is frequently used to manufacture semiconductor devices and integrated circuits (e.g., semiconductor logic products). During lithography a radiation sensitive layer or photoresist on a semiconductor device is selectively exposed to radiation through the use of a mask or reticle. Based on the exposure, select portions of a silicon wafer are exposed for subsequent processing associated with creating circuitry patterns.
FIG. 1 illustrates typical components of a lithography system 100. A radiation source 110 generates and transmits radiation 120 towards a radiation sensitive layer 160, through a mask 130 that contains a pattern 140. The mask 130 and the pattern 140 may have transparent regions that selectively transmit portions of the radiation 120 and opaque regions that selectively block portions of the radiation 120. Typically, the pattern 140 consists of alternating regions of opaque chrome on transparent quartz. The mask 130 and the pattern 140 selectively allow radiation 150 to expose a portion 170 of the radiation sensitive layer 160. After exposure conventional development processing may be used to remove a portion of the radiation sensitive layer 160. In a positive-development process the exposed portion 170 may be transformed so that it is comparatively easy to remove by development, while in a negative-development process the exposed portion 170 may be transformed so that it is comparatively difficult to remove. After development, conventional processing may be used to create circuitry related structures in the wafer 180.
The wavelength of the radiation 120, 150 affects the size of the circuitry that can be produced by lithography. Shorter wavelengths allow smaller features to be produced. Traditional wavelengths that have been used include 436 nm (called G-Line), 405 nm (H-line), 365 nm (I-line) and 248 nm (called Deep Ultraviolet), 193 nm, 157 nm, and others. The shorter wavelengths below about Deep Ultraviolet may typically result in a sub-wavelength condition, in which features to be created have smaller dimension than the wavelength of the radiation used to create them. This condition may introduce unintended and undesirable distortions and inaccuracies. For example, there may be optical proximity error that causes placement and fidelity of a first feature to be distorted by a neighboring feature.
FIG. 2 illustrates a mask 210 having a simple rectangular chrome pattern 220 having a width and a length. In a sub-wavelength condition, wherein at least one of these dimensions is shorter than a wavelength of radiation used to print or expose the pattern 220 into a radiation sensitive layer, the exposed pattern may have unintended and undesirable distortions and inaccuracies.
FIG. 3 illustrates a radiation sensitive layer 310 of a wafer (not shown) having an exposed feature 320 that differs from the pattern 220 due to subwavelength effects such as optical proximity error. In this case, since the pattern 220 was chrome, the exposed feature 320 is an unexposed region. A dashed line indicates an intended exposure feature 330 having dimensions substantially identical to the pattern 220. As shown, the actual exposed feature 320 differs from the intended exposure feature 330. In particular, a length of the exposed feature 320 is less than a length of the intended exposure feature 330. This represents line shortening. Additionally, the exposed feature 320 has rounded corners while the pattern 220 and the intended exposure feature 330 have squared corners. This represents corner rounding. These irregularities and inaccuracies can reduce the performance of circuits based on the feature 320 and/or cause them to fail.
FIG. 4 illustrates a mask 410 having an optical proximity corrective chrome pattern 420 that performs optical proximity correction (OPC) to the rectangular pattern 220 to reduce unintended irregularities and inaccuracies due to subwavelength effects like the optical proximity error. This may improve pattern fidelity by allowing an exposed feature generated from the pattern 420 to correspond more closely with an intended exposure feature. However, this approach offers a number of challenges. A first challenge is that the pattern 420 is complex and difficult to fabricate. A second challenge is that it becomes difficult to simultaneously provide such complex corrective features and simultaneously reduce feature dimensions, which may both be desired.
FIG. 5 illustrates a passive trim mask 510 having a protective chrome pattern 520. The passive trim mask 510 and the protective pattern 520 correspond to the mask 410 and are used to completely protect an exposed feature corresponding to the pattern 420 during a second exposure to radiation. The protective pattern 520 is larger than the pattern 420. Accordingly, the second exposure to radiation does not affect, modify, or alter a feature exposed in a radiation sensitive layer by a first exposure using the pattern 420.